Low latency analog QAM coherent demodulation algorithm

ABSTRACT

A digital demodulation method for a quadrature amplitude modulated signal uses a phase locked loop to generate a local carrier signal. The phase locked loop uses a feedback signal derived from one or more demodulated signals of interest. The loop has a filter characteristic with a stop band within the information bandwidth(s) of the information signal(s). The preferred method generates an error signal from DC components of in-phase and quadrature-phase baseband signals. DC components are preferably isolated using a low-latency, AC rejection filter.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in the invention and the rightin limited circumstances to require the patent owner to license otherson reasonable terms as provided for by the terms of contract P-7038(restricted customer).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is useful in the field of radio-frequency (RF)transmission, and in particular to digital demodulation of quadraturephase amplitude modulated RF signals.

2. Discussion of Background Information

Certain techniques for amplitude modulation and demodulation of a radiofrequency carrier are known. FIG. 1 illustrates a demodulation methodfor an amplitude modulated (AM) signal using digital down converters. Anincoming amplitude modulated RF 10 signal is filtered 12 and quantized14 to form a quantized AM signal 16. The quantized AM signal 16 isdownconverted 18 to form an in-phase baseband signal 20 which may below-pass filtered 22. The quantized AM signal 16 is also downconverted24 to form a quadrature-phase baseband signal 26 which may be low-passfiltered 28. A phase lock loop (PLL) circuit uses a numericallycontrolled oscillator (NCO) 34 to generate a local carrier signal 36that tracks the carrier phase of quantized AM signal 16. The localcarrier signal 36 is used to downconvert the quantized AM signal 16 intothe in-phase baseband signal 20 using a well-known multiplicationrelationship. The carrier signal 36 is also phase shifted 38 and used todownconvert the AM signal 16 into the quadrature-phase baseband signal26 using a well-known multiplication relationship. One baseband signal,such as a quadrature-phase baseband signal 30, is feed directly to a PLLfilter 32 for control of the NCO 34. Conventional AM demodulationmethods assume that the pass band of the PLL filter is larger than thehighest frequency signal of interest.

The demodulation method illustrated in FIG. 1 relies on a well-knownalgebraic approximation that sin (x)≈x for small x. In the method ofFIG. 1, the NCO would ideally be controlled by the sine of the errorbetween the NCO-generated carrier phase 36 and the carrier phase of thequantized AM signal 16. For simplification, the estimation signal 30used to control the NCO is the baseband signal. Under thisapproximation, the quadrature phase baseband signal goes to zero whenthe detector is ideally locked. Additional background information onphase lock loops AM demodulation can be found in H. Meyr,Synchronization in Digital Communications & Digital CommunicationsReceivers, John Wiley & Sons, Inc. New York, 1998.

SUMMARY

In prior AM demodulation methods, the presence of a quadrature datasignal can degrade PLL performance. For example, in the demodulationmethod of FIG. 1, the quadrature-phase baseband signal 26 would containinformation for a second data channel. The filtered signal 30 thereforewould not go to zero, even when the NCO 34 is ideally locked. Thepresence of information on the quadrature channel can degrade PLLperformance and cause false locks. The novel demodulation methoddescribed here can be used to replace prior AM demodulation equipment inhigh-channel-count digital receiver hardware.

An object of the invention is to provide demodulation of analog QAMsignals where both an in-phase and a quadrature-phase are used totransmit acoustic data channels.

A further object of the invention is to decrease channel cross-talkbetween in-phase and quadrature-phase channels (as compared toconventional digital PLL demodulation).

A further object of the invention is to avoid false locks in the PLL ofa QAM demodulator.

A further object of the invention is to provide for demodulation ofdouble-side-band, reduced carrier AM.

A further object of the invention is to provide for AM demodulation withlow latency in the phase-locked loop.

These and other objects are achieved by providing a low-latency methodfor demodulation of QAM signals where both the in-phase andquadrature-phase channels are used for transmission of data channels andit is desirable to have high channel separation (low cross-talk). Asignal decomposition process removes some or all of the non-zerofrequency signal components prior to phase and frequency estimation.This minimizes PLL degradation due to data on the quadrature-phasechannel. A one-pole AC rejection filter in the phase-locked loop reduceslatency compared to a baseband, low-pass FIR filter. A signal estimationsection uses both the in-phase and quadrature-phase signals forcalculating values of phase and frequency error.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the noted plurality of drawings by way ofnon-limiting examples of certain embodiments of the present invention,in which like numerals represent like elements throughout the severalviews of the drawings, and wherein:

FIG. 1 illustrates a conventional demodulation method for an amplitudemodulated (AM) signal using digital downconverters;

FIG. 2 illustrates a novel demodulation method;

FIG. 3 illustrates signal decomposition;

FIG. 4 illustrates signal estimation;

FIG. 5 illustrates a filter.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the several forms of thepresent invention may be embodied in practice.

Equation 1 describes a QAM signal.

s(t)=[A+mx ₁] cos(ω_(c)+φ_(c))+mx ₂ sin(ω_(c)+φ_(c))  Equation 1

where:

A=Carrier amplitude

o)=Carrier Frequency in radians

(p=Carrier phase

m=Modulation index

x₁=Channel 1 signal

x₂=Channel 2 signal

Channel signals x₁ and x₂ may be acoustic channels of interest wherehigh channel separation (low cross-talk) is desirable.

One approach to solving the problem of the x₂ signal corrupting thephase error estimate would be to modify the FIG. 1 method by making theloop filter 32 a low-pass, Finite Impulse Response (FIR) filter with acut-off frequency low enough to eliminate the quadrature informationsignal. This approach does improve channel separation. The amount ofimprovement is limited because the feed back loop is sensitive tolatency. Lowering the cut-off frequency of the low pass filter requiresincreasing the length of the filter to potentially thousands of taps,which increases latency.

A preferred method described below solves both the problem of latency inthe phase-locked loop and the problem of signal content on thequadrature-phase channel. First the in-phase and quadrature-phasebaseband signals are decomposed into their zero frequency (DC) andhigher frequency (AC) components. Then, the DC signals are used toestimate frequency and phase errors for the PLL. To avoid the problem ofintroducing latency into the feed back loop, the signal decompositionalgorithm is based on a one-pole AC rejection filter. The method may beused on fixed point Digital Signal Processors (DSPs) with noise shapingintegrated into the signal decomposition algorithm.

FIG. 2 illustrates the more preferred demodulation method. An incomingamplitude modulated RF signal 40 is filtered 42 and quantized 44 to forma quantized AM signal 46. The quantized AM signal 46 is downconverted 48to form an in-phase baseband signal 50. The quantized AM signal 46 isalso downconverted 54 to form a quadrature-phase baseband signal 56. Aphase lock loop (PLL) circuit uses a numerically controlled oscillator(NCO) 64 to generate a local carrier signal 66 that tracks the receivedcarrier. The local carrier signal 66 is used to downconvert thequantized AM signal 46 into the in-phase baseband signal 50. The carriersignal 36 is phase shifted 68 by ninety degrees and used to downconvertthe AM signal 46 into the quadrature phase baseband signal 56.

The in-phase baseband signal 50 is decomposed 70 into a DC component 72and an AC component 74. The quadrature-phase signal 56 is alsodecomposed 76 into a DC component 78 and AC component 80. The ACcomponents are the in-phase (I) and quadrature-phase (Q) data channels.DC components 72, 78 are used to estimate phase and frequency errors 82for every sample cycle. The phase error 86 and frequency error 84 areboth filtered 88, 90, and one or the other selected 92 for control ofthe NCO 64. The filtered frequency error may be used during pull-in oracquisition operation when the PLL is attempting to lock to the incomingcarrier. The filtered phase error may be used after frequency lock hasbeen achieved. Signal decomposition 70, 76, error estimation 82, andfiltering 88, 90 will be discussed in more detail below.

FIG. 3 illustrates signal decomposition. An input signal X may be anin-phase or quadrature-phase baseband signal 50, 56 as illustrated inFIG. 2. Output signal y_(ac) is the AC component of input signal X.Output signal y_(dc) is the DC component of the input signal X. “+”indicates an addition function. “X” indicates a multiplication function.Z⁻¹ is a delay block. ε indicates a constant used to adjust the dynamicstability (e.g., settling time) of the filter. The output of the thirdtap is fed back after a delay to the second tap, which provides noiseshaping. Such a filter has a latency of less than hundreds or even tensof samples, and in any event less than thousands of samples of a typicalFIR filter. The filter transfer function is given by:

x1[n]=x[n]−x[n−1]

y[n]=floor(pole*y[n−1]+x1[n]−err[n−1])

err[n]=y[n]−(pole*y[n−1]+x1[n]−err[n−1])

so:

y_(ac)=y[n]

y _(dc) =y[n]−x[n]

Filter cut-off frequency may be adjusted to achieve desired channelseparation and other characteristics, such as pull-in time. It isexpected the cut-off frequency will be lower than the pass-band of thesignal of interest. For acoustic signals of interest, the cut-offfrequency could be on the order of tens of hertz or ones of hertz, suchas 2 hertz. Cut-off frequency may be described using any known filtercharacterization methods, such as determining the frequency at which thefilter's amplitude response falls 3 decibels (dB).

FIG. 4 illustrates signal estimation. I and Q channel DC values ofI_(dc) and Q_(dc) for one sample define a point in the complex planehaving a phase angle φ relative to the positive x-axis. The sine andcosine of the phase angle can be calculated from the DC values I_(dc),Q_(dc). The inverse tangent of the sine/cosine ratio is the phase angleφ. A change in phase angle from one sample to another sample Δφ can becomputed from sequential values of phase angle φ. A time derivativedΔφ/dt of the change in phase angle Δφ can be calculated using numericaldifferentiation. The phase error and frequency error are given by theequations:

Frequency Error=d(Δφ)/dt.

Phase Error=tan⁻¹(Δφ)[sin(Δφ)/cos(Δφ)].

FIG. 5 illustrates a loop filter. Symbology is the same as for FIG. 3.An input signal x may be the frequency error 84 or phase error 86 fromsignal estimation 82 (FIG. 2). An output signal y is a filtered versionof the input signal x. C_(i) and C_(p) are filter coefficients. Thetransfer function is given by the equation:

y[n] = C_(p)x[n] + C_(i)x[n − 1] + y[n − 1]$C_{p} = {\frac{T_{s}}{2\tau_{1}}\left\lbrack {1 + \frac{1}{\tan \left( \frac{T_{s}}{2\tau_{2}} \right)}} \right\rbrack}$$C_{i} = {\frac{T_{s}}{2\tau_{1}}\left\lbrack {1 - \frac{1}{\tan \left( \frac{T_{s}}{2\tau_{2}} \right)}} \right\rbrack}$where:$\tau_{1} = {{\frac{k_{0}k_{d}}{\omega_{n}^{2}}\tau_{2}} = \frac{2\zeta}{\omega_{n}}}$

The same filter structure may be used for both the phase and frequencyloop filters. Coefficients may be individually optimized to achievedesired pull-in and lock time responses. The gain constant k₀k_(d) maybe set with ζ equal to 0.707 to give a one second settling time.

Signal decomposition 70,76, signal estimation 82, and error filtering88,90 can all be implemented on a single DSP chip, such as a 300 MHz TI6203 with DSP bios. In fact, multiple instances of these processes maybe implemented on a single DSP chip. Where more than two channels ofdata require transmission, multiple demodulators may be implemented.Multiple demodulator instances may be synchronized by inserting a pilotsignal, such as the carrier, into a QAM channel and further adjustingNCO's of different demodulators until pilot tones from all demodulatorsare synchronized. Multi-channel synchronization is sometimes calledcoherent demodulation.

The embodiments disclosed above downconverted information channels tobaseband and derived an error signal from DC components of the basebandsignals. It should be appreciated that the information channels could bedownconverted to intermediate frequencies, and that a variety of filtercharacteristics may be used having stop bands within the informationbands of the channels used to derive the error signal.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to certain embodiments, it is understood that the wordswhich have been used herein are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present invention in itsaspects. Although the present invention has been described herein withreference to particular means, materials and embodiments, the presentinvention is not intended to be limited to the particulars disclosedherein; rather, the present invention extends to all functionallyequivalent structures, methods and uses, such as are within the scope ofthe appended claims

1. A digital demodulation method for a QAM signal carrying aninformation signal of interest comprising: downconverting theinformation signal of interest from the QAM signal using a local carriersignal; using a phase-locked loop to generate the local carrier signalin response to a feedback signal derived from the downconverted signalof interest, said phase-locked loop having a filter characteristic witha stop band within the information bandwidth of the information signalof interest.
 2. The method of claim 1 wherein the step of using aphase-locked loop comprises a step of estimating a phase error betweenthe QAM carrier and the local carrier using a DC component of thedemodulated signal of interest.
 3. The method of claim 1 wherein thefilter characteristic has a pass band between about zero hertz and abouttens of hertz in the information signal bandwidth.
 4. The method ofclaim 1 wherein the filter characteristic is a low pass filter with acut-off between about zero hertz and about ones of hertz in theinformation signal bandwidth.
 5. The method of claim 1 wherein thefilter characteristic is a low pass filter with a cut-off between aboutzero hertz and about two hertz in the information signal bandwidth. 6.The method of claim 1 wherein the phase-locked loop includes an ACrejection filter having a stop band within the information bandwidth ofthe information signal of interest.
 7. The method of claim 1 wherein thephase-locked loop filter includes a low-pass filter having a stop bandwithin the information bandwidth of the information signal of interest.8. The method of claim 1 wherein the filter characteristic isimplemented with a latency of less than about thousands of samples. 9.The method of claim 1 wherein the filter characteristic is implementedwith a latency of less than about hundreds of samples.
 10. The method ofclaim 1 wherein the filter characteristic is implemented with a latencyof less than about tens of samples.
 11. A digital method of demodulatinga QAM signal having first and second information signals modulated onin-phase and quadrature phase channels of a carrier, the methodcomprising: forming a first information signal by downconverting the QAMsignal using a first locally-generated carrier signal; forming a secondinformation signal by downconverting the QAM signal using a secondlocally-generated carrier signal that is out of phase with firstlocally-generated carrier signal; isolating DC components of the firstand second downconverted signals, said DC components excluding at leastsome frequency components in the information bandwidths of theinformation signals; deriving an error signal from the isolated DCcomponents; and adjusting a phase of a locally-generated carrier signalaccording to the error signal.
 12. The method of claim 11 wherein thestep of isolating DC components includes a step of excluding frequencycomponents above about tens of hertz in an information signal bandwidth.13. The method of claim 11 wherein the step of isolating DC componentsincludes a step of excluding frequency components above about ones ofhertz in an information signal bandwidth.
 14. The method of claim 11wherein the step of isolating DC components includes a step of excludingfrequency components above about two hertz in an information signalbandwidth.
 15. The method of claim 11 wherein the step of isolating DCcomponents is implemented with a latency of less than about thousands ofsamples.
 16. The method of claim 11 wherein the step of isolating DCcomponents is implemented with a latency of less than about hundreds ofsamples.
 17. The method of claim 11 wherein the step of isolating DCcomponents is implemented with a latency of less than about tens ofsamples.
 18. The method of claim 11 wherein the step of isolating DCcomponents is implemented with an AC rejection filter.
 19. A demodulatorfor a digitized QAM signal carrying first and second informationsignals, the demodulator comprising: a first downconverter generating afirst downconverted information signal in response to the QAM signal anda local carrier signal of a first phase; a second downconvertergenerating a second downconverted information signal in response to theQAM signal and a local carrier signal of a second phase and; anoscillator generating the local carrier signal in response to an errorsignal; wherein the error signal is derived from components of the firstand second downconverted information signals, said components includingDC components but attenuating AC components in the information signalinformation bandwidths.
 20. The demodulator of claim 19 wherein theerror signal is derived by isolating DC components from AC componentsusing an AC rejection filter;
 21. The demodulator of claim 19 whereinthe error signal is derived by isolating DC components from ACcomponents at a cutoff above about tens of hertz in the informationsignal bandwidth.
 22. The demodulator of claim 19 wherein the errorsignal is derived by isolating DC components from AC components at acutoff above about ones of hertz in the information signal bandwidth.23. The demodulator of claim 19 wherein the error signal is derived byisolating DC components from AC components at a cutoff above about twohertz in the information signal bandwidth.
 24. The demodulator of claim19 wherein the DC components are derived from the downconvertedinformation signals with a latency of less than about thousands ofsamples.
 25. The demodulator of claim 19 wherein the DC components arederived from the downconverted information signals with a latency ofless than about tens of samples.